1. Field of the Invention
The present invention relates to a process for the production of particulate, propellant-containing styrene polymers. More particularly, the present invention relates to a process for producing propellant-containing styrene polymers having a reduced residual monomer content as well as a distinctly lower benzene concentration than prior art materials. The invention also relates to the product propellant-containing particulate polymers themselves, foamed particles thereof and moldings comprising these polymers.
2. Discussion of Background
Particulate foams based on styrenic polymers have achieved considerable industrial importance as packaging and heat insulating materials. These foams are produced on an industrial scale by suspension polymerization of styrene in the presence of a propellant. During processing, the particulate expandable styrene polymer (EPS) is first foamed by applying heat to give foamed particles which may then be welded in closed molds to give moldings.
The suspension polymerization of styrene to produce expandable styrene polymers is usually carried out in a discontinuous process which uses two different temperature steps and two different peroxide polymerization initiators having different half-lives. Usually, dibenzoyl peroxide is used during a first polymerization step at 80.degree. to 90.degree. C. and tert-butyl perbenzoate, which decomposes at a higher temperature than dibenzoyl peroxide, is used during a second polymerization step at 105.degree. to 130.degree. C. in order to provide a product having a residual monomer content which is as low as possible.
Unfortunately, the expandable styrene polymers produced by this process contain benzene in concentrations of 0.002 to 0.01 percent by weight, typically 0.003 to 0.005 percent by weight. While the benzene can be liberated by heat treatment of the styrene polymers during treatment processes, the benzene concentration in the finished product is still from 0.0005 to 0.001 percent by weight.